Method for destruction of toxic substances with ultraviolet radiation

ABSTRACT

A method for destruction of toxic compounds through direct ultraviolet irradiation. Continuum UV radiation from the deep region of the spectrum is applied to the target medium in pulsed fashion with a specified range of power ratios and average power. Enhanced destruction of undesirable toxins is achieved when the ratio of rms power to average power falls in characteristic range of 50:1 to 200:1; the ratio of peak power to average power falls in the characteristic range of 1000:1 to 10,000:1; and the average power density is maintained at least at a value of about 0.5 Watt/cm 2  within the carrier medium.

This is a continuation-in-part of application Ser. No. 07/549,506, filedJul. 6, 1990, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to the destruction of toxic substancessuch as organic compounds and microbial species in the process ofpurification or disinfection of aqueous and other environments.

Various undesirable compounds, such as heavy organic molecular compoundsand microbial species, are often carried in waste water or othereffluents, soils or other matrix environments, in which they may provetoxic in subsequent uses of the carrier material. One known process forsterilizing or disinfecting the carriers of these compounds is throughirradiation with ultraviolet (UV) radiation. Most chemical bonds inorganic toxins are broken under the action of the ultraviolet radiationthrough photodissociation. A particular substance will have acharacteristic photodissociation curve associated with it specifying theenergies of UV radiation for which the particular substance will undergophotodissociation. For effective photodissociation it is necessary thatthe UV radiation have the particular energy or energies which fallwithin the photodissociation curve of the substance of interest. Formost organic toxins of interest here the photodissociation curves aregreatest (indicating the greatest likelihood of dissociation) in therange of 175 to 300 nanometers (nm).

Most dissociation curves of interest have an effective range which caninclude many discrete UV emission energies (so-called emission "lines").For effective destruction of the undesirable compounds it is not enoughthat the UV energies fall in the applicable range. Another necessarycondition is that the radiation have a sufficient intensity. Thenecessary intensity depends on the photodissociation cross sections forthe undesirable compounds and their concentrations in the carriermedium.

There is also a problem in effective destruction of toxic compounds inthat the photodissociation process for a given compound may produceby-products which are themselves toxic and which must undergo furtherphotodissociation until non-toxic end-products result. In other words acascade of consequent UV photodissociation actions has to be applied tothe original toxins and to the possibly toxic byproducts of the UVactions until the final byproducts are reduced to safe substances.

Typical UV sources generate only a relatively few intense UV emissionlines falling in this energy range. These relatively few intense linesdo not generally fall within the peak absorption range for all of thetoxic compounds and their photodissociation byproducts occurring in atypical specimen.

Direct UV destruction of toxic compounds has not been employed in theprior art, which instead has relied on UV excitation of knownintermediate additives such as ozone or peroxides.

SUMMARY OF THE INVENTION

The present invention provides a method for breaking down toxiccompounds of the above sort into their final non-toxic end-productsthrough direct ultraviolet irradiation. The invention achieves thisresult by applying sufficiently intense UV radiation from the deepregion of the UV spectrum in a sufficiently broad band to form aneffective continuum and thereby overlap with the absorption curves foressentially any of the toxins of interest and of their toxic byproducts(if any) from such UV photodissociation. More particularly, it has beenfound that effective destruction of toxins with concentrations rangingfrom 200 parts per million (ppm) to 10 parts per billion (ppb) can beachieved by delivering the deep UV radiation to the target medium inpulsed fashion where the root-mean-square (rms) and peak power deliveredby the UV source are related to the average power by characteristicratios, while the average power density is at least a minimumcharacteristic value. The precise values of the characteristic ratiosand power densities that optimize the destruction of the target toxinsdepends upon the concentrations and the carrier medium of the toxins.Nevertheless, it has been found that the surprisingly effectivedestruction achieved by the invention will generally be attained whenthe ratio of rms power to average power falls in the characteristicrange of 10:1 to 100:1; the ratio of peak power to average power fallsin the characteristic range of 1000:1 to 10,000:1; and the UV averagepower density is maintained at least at a value of about 0.1 Watt/cm²within the carrier medium. The characteristic profile of the UVradiation provided according to the invention is such that it can beachieved within the bounds of a practical instrument.

Other features and advantages of the invention will be described belowor will be apparent to those skilled in the, art from the followingdescription and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of apparatus for practicing the invention.

FIG. 2 is a block schematic diagram of the control unit and lamp of FIG.1.

FIGS. 3A and 3B show comparative test results for the process accordingto the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention calls for subjecting the toxic species desired tobe destroyed to ultraviolet radiation under special conditions, whichare indicated below. By way of illustration the invention is describedhere as applied to the purification of waste water. Those skilled in theart will appreciate from the present disclosure, however, that theinvention may be applied to destroy undesirable species in a variety ofother environments as well.

Apparatus for subjecting the water under treatment to ultravioletradiation is well known to persons skilled in the art and need not bedescribed here in any detail. For simplicity, therefore, the apparatusis illustrated only diagrammatically in FIG. 1. For treatment ofmaterials other than waste water it will be necessary of course toemploy apparatus suitable for the particular material under treatment toexpose the material to a source of UV radiation.

FIG. 1 shows a processing chamber 10 through which the water undertreatment flows in the direction indicated by the arrows 11. Embeddedwithin chamber 10 is a source 12 of UV radiation. Although illustratedas embedded within chamber 10, the UV source 12 may, of course, also besituated outside the chamber and irradiate the water under treatmentthrough quartz windows transparent to UV radiation. UV source 12 isdriven by control unit 13.

It has been found that highly effective purification can be achieved fora wide range of toxins if the UV source is caused to provide anirradiating beam, operated in a pulsed mode, and having a characteristicfrequency profile as well as a characteristic power profile. Thefrequency spectrum of the individual pulses includes UV radiation in aband in the wavelength range of 175 to 360 nm. The band may comprise acontinuous spectrum or at least a band of discrete emission lines ofsufficiently high density approximating a continuous spectrum. Theprocess as described is advantageous in part because the UV radiation isdelivered in an intense continuum which overlaps the absorption curvesof most toxins. Thus, the destructive power of the process is notlimited to a particular toxic substance arising in a particularapplication, but is effective against a broad range of substances andtheir toxic byproducts generated when the primary toxin is broken downby a cascade of consequent photolytic actions of the pulsing UVcontinuum.

Known methods in the past have avoided the use of a continuous spectrum,instead employing UV radiation at specific available frequencies, suchas the frequencies of the basic mercury lines or other available linesin metal halide lamps. Furthermore, radiation at the frequencies ofthese lines are typically used to activate intermediate agents such asozone or peroxide. These methods lack sufficient power at thecharacteristic frequencies for breaking down the toxins directly.

It has been discovered in the present invention that the abovelimitations may be overcome and a great enhancement in the destructivecapability of the UV radiation may be achieved by adjusting the peakpower, the rms power, and the pulse rate and average power at which theradiation is delivered to the medium while providing at least a minimumpower density level for the destruction of the toxic concentrations ofinterest in any given example. The precise values of these parameters inany given case depends upon, and can be calculated from, thephotodissociation cross sections and the concentrations of the targettoxins. The techniques for performing such calculations are well knownin the field of UV absorption spectroscopy and the necessary crosssections, for the most part, have been measured and may be found in theavailable technical literature. Nevertheless, such calculations may betedious, and as a practical matter, it will be simpler to determine theoptimal values of the parameters in any given case empirically. Forexample, as a baseline for comparison a sample under treatment may becontinuously exposed to radiation from a mercury lamp for, say, fiveminutes, and the resulting reduction in concentration of the toxicsubstances measured. An equivalent sample may then be exposed to apulsed beam at the same average power level and the resultingdestruction of toxins again measured. The peak power and rms power arethen adjusted, e.g., by adjusting the current and rise time in the UVlamp, and the variation in concentrations noted. Then the pulserepetition rate may be adjusted and the change noted. In this manner theapproximate optimum values of the system parameters may be determinedempirically with a relatively few iterations.

While the precise values optimizing performance will generally depend onthe specific target medium and toxins, nevertheless, it has been foundthat the surprisingly effective destruction achieved by the presentinvention will generally be attained when the ratio of rms power toaverage power falls in the characteristic range of 10:1 to 100:1; theratio of peak power to average power falls in the characteristic rangeof 1000:1 to 10,000:1; and the average power density is maintained atleast at a value of about 0.1 Watt/cm² within the carrier medium whilethe UV source is pulsed at a repetition rate in the range of 5 to 100Hertz (Hz). These ratios assure that the processes by which the bonds inthe toxins are broken are far more prevalent than the competing reverseprocesses. The peak power may of course be greater than indicated in theabove ratio and a greater average power than indicated above will, ofcourse, be even more beneficial.

For example, an average power, rms power, and peak power in the relativeproportions of:

average power:rms power:peak power=1:20:2000 has been found to beeffective for concentrations, of volatile and semi-volatile toxins inthe range between 100,000 and 50 ppb. For oil and grease at aconcentration on the order of 10,000 ppb the effective proportions werefound to be 1:10:1000.

In this way, a variety of toxins may be effectively treated withouthaving to tune or select the UV source specifically to the toxins ofinterest and consequently without even having to know in advance thecomposition of toxins present. If, however, the combination of toxicsubstances present in the sample under treatment should be known inadvance, then the absorption curves of the individual substances may bedetermined through known UV absorption spectroscopy techniques and thesource parameters adjusted to match the principal portion of theabsorption curves with the principal portion of the continuum from theUV. For example, if the toxins present are known to have absorptioncurves in the limited range of, e.g., 200 to 240 nm, then the processefficiency may be enhanced by tailoring the output of the UV source tothis range, i.e., by adjusting the source parameters to increase theoutput in this range and to minimize the output outside of this range.

The pulse rate of the source may also be adjusted according to the toxinconcentration and sample flow rate. For example, for contaminated waterpassing through the system at a flow rate of 80 gallons per minute(gal/min), the pulse rate may be set at 40 pulses per second. If theflow rate should slow to 64 gal/min, then the pulse rate may follow thechange in flow rate and be slowed proportionately to 32 pulses persecond.

With reference now to FIGS. 1 and 2, the process according to thepresent invention may be practiced with a UV source 12 provided by axenon flash lamp having a high quality quartz (suprasil) envelope. Thelamp may be driven with the power supply and switching circuitarrangement shown in the block schematic diagram of FIG. 2, whichincludes a power supply and pulse control unit 16, charging capacitor17, silicon controlled rectifier (SCR) 18, simmer power supply 19,ignition wire 21 and xenon flash lamp 22. Charging capacitor 17typically has a capacitance on the order of 4 microfarads and powersupply 16 provides a charging voltage in the range of 2 to 5 kV. Thenetwork operates to produce a small (2 to 3 Ampere) DC current throughlamp 22 (so-called simmer current). The voltage across lamp 22corresponding to this DC current will typically be 70 to 200 Volts oncethe simmer current is established. To establish the simmer current, aninitial voltage of about 1 kV DC is applied to the lamp and the lamp isignited with a high voltage spark through ignition wire 21. As soon asthe simmer current is established, SCR 18 can be opened periodically todischarge the capacitor into lamp 22. With this arrangement and a lamphaving a bore diameter of 6 mm, a peak current of 1,500 Amperes can bereached with a rise time of about 7 microseconds. In general, with axenon flash lamp a pulsed peak current density J of 4 to 8 kiloamperesper square centimeter (kA/cm²) and a rate of rise dJ/dt of 200 to 500amperes per square centimeter per microsecond (A/cm² -μsec) may beachieved.

The process described herein has been demonstrated to break down majororganic toxins directly into simple, non-toxic end-productssignificantly more effectively than could be achieved with a standard DCmercury lamp with the same average electrical power input. FIGS. 3A and3B show the results of sample test runs. The test data were achievedusing a xenon flash lamp driven under the conditions described above.

FIG. 3A shows the photolytic destruction of a sample of emulsified oilusing the pulsed continuum UV radiation with power ratios as describedabove. The curve labeled DC shows the reduction of oil contaminants inthe sample achieved with a lamp run continuously in the conventionaldirect current mode. The curve labeled PL shows the reduction in thesame sample using the "pulsed light" mode according to the presentinvention.

FIG. 3B shows a similar test performed with a sample contaminated bytrichloroethylene. The curves labeled DC show the destruction of thecontaminant by UV irradiation using a medium pressure mercury lampwithout the benefit of the present invention. In one DC curve the sampleis merely irradiated. In the other DC curve peroxide has also been addedto the sample. The curves labeled PL show the improvement achieved inboth cases by means of the present invention. The PL curves weregenerated with power ratios of

    average:rms:peak=1:20:2000;

the average flux density over the sample was

    P.sub.Av =0.40 W/cm.sup.2

in the UV region. Note that at 50 Watt-hours of applied electricalenergy, the destruction is about 100 times more effective than theaction with the mercury lamp.

The above provides a full and complete disclosure of illustrativeembodiments of the present invention. Given the benefit of thisdisclosure, various alternate embodiments, modifications, andequivalents will occur to those skilled in the art. For example, avariety of arrangements may be employed to irradiate the material undertreatment, and a variety of lamps and power supply designs may beconfigured to provide the pulse, frequency, and power profiles calledfor by the invention. The process of the present invention is believedto achieve its destructive results principally through directphotodissociation of the target toxins into safe final byproducts by acascade of photolytic actions of a pulsing UV continum applied toinitial toxins and their toxic byproducts (if any). This cascade processof consequent photolytic actions is effectively sustained due to thefact that the pulsing UV continuum with an established range of average,rms and peak ratios has the most effective photolytic action on any ofthe original toxins and on any of their photolytic byproducts if thelatter are also toxic. The process can be terminated when all photolyticbyproducts are reduced to safe substances. It will thus be appreciatedby those skilled in the art that the process will be applicable not onlyto toxins in aqueous solution, but also to toxins in any environment inwhich the toxins are free to undergo direct photodissociation.Accordingly, the invention is not intended to be limited only to thespecific examples and embodiments disclosed herein, but is defined bythe appended claims.

What is claimed is:
 1. A process for the destruction of toxic substancesin a medium by means of ultraviolet radiation in which the medium issubjected to a band of ultraviolet radiation of high spectral density,said band lying within the wavelength range of 175 to 360 nanometers,said ultraviolet radiation being pulsed at a pulse repetition rate of atleast about 5 Hertz, in which the ratio of root-mean-square power toaverage power of said ultraviolet radiation lies in the range of 10:1 to100:1, and the ratio of peak power to average power delivered by saidultraviolet radiation lies in the range of 1,000:1 to 10,000:1, saidultraviolet radiation having an average power density of at least about0.1 Watt/centimeter².
 2. The process of claim 1 wherein said band ofultraviolet radiation of high spectral density is provided by acontinuous band of ultraviolet frequencies.
 3. The process of claim 1wherein said band of ultraviolet radiation of high spectral density isprovided by a dense band of discrete ultraviolet emission lines.
 4. Aprocess for the destruction of toxic substances by photodissociationcomprising the steps of:providing an extended spectral band ofultraviolet radiation lying within the range of 175 to 360 nanometers;pulsing the ultraviolet radiation at a pulse repetition rate of at leastabout 5 Hertz, the pulsed extended spectral band of ultravioletradiation having a characteristic rms power ratio of rms power toaverage power, a characteristic peak power ratio of peak power toaverage power, and a characteristic average power density; and adjustingsaid pulse repetition rate, said rms power ratio, said peak power ratio,and said average power density relative to one another to maximizedestruction of said toxic substances and toxic byproducts thereof.